Creature feature

In 1979, director Ridley Scott terrified moviegoers with his horror classic about an alien that gestates in a human host. Since then, the Alien film series has earned a venerable reputation in science fiction by adhering to a simple structure: pushing its heroine, Ripley, to her physical and emotional limit against the insect-like, metallic monster in frightening action and suspense sequences. For the latest chapter in the story, Alien vs. Predator (AvP), director Paul W. Anderson strayed from that formula, and borrowing a page from the Dark Horse Alien vs. Predator comic-book scripts, locked two of moviedom's iconic alien antagonists—the biometallic Alien and the camouflaged hunter Predator—in an epic battle.

In the setup to this monster clash, a team of unsuspecting scientists and adventurers arrive in Antarctica to investigate an ancient pyramid. After discovering that Predators are keeping alive a captive Alien Queen, whose offspring are used to test the prowess of young Predator warriors, the group finds itself battling for survival in the midst of this inter-alien warfare. While critics dismissed the film as gimmickry, and purists feared it could portend the end of both series, AvP raked in over $38.3 million at the box office during its opening weekend.

The characters in AvP appear in close-up shots, which required the artists to create believable CG versions of both the Aliens (left) and the Predators (right) that had the same mystique as their obscured predecessors that appeared in earlier films

In spite of the less-than-positive reviews, the film received acclaim for the CG character animation completed by The Moving Picture Company (MPC), which created the digital creatures and seamlessly integrated them into the live action with a clarity and visibility never before achieved in the previous Alien films. While animatronic and cable-operated puppets were often used in the past, the closer shots of the larger Aliens were reserved for a person in a suit, so rarely, if ever, were they shown in plain view, so as to not betray the human form inside. That "less is more" strategy proved effective and was carried over into the subsequent films, until now.

"For AvP, Anderson wanted the Aliens unobscured, fully active, and credible, but with the same mystique from the older Alien films," explains animation director Adam Valdez, who also served as visual effects supervisor. This meant that MPC had to fight its battle on two fronts. Not only did the artists have to populate the Antarctic pyramid site with some of the most believable CG Aliens yet created, but they also had to digitally enhance full-size animatronics to give them far greater movement and exposure than before.

Amalgamated Dynamics, Inc. (ADI) created the conceptual designs for AvP's Predators, the Alien Queen, and her warriors according to Anderson's mandate of remaining faithful to their previous incarnations. Of particular focus were those from James Cameron's 1986 Aliens, which included the rod- and cable-operated "face huggers" and "chest busters," the lightweight bodysuits that gave the creatures their insect-like agility, and the 14-person-operated queen. On set, however, the group was unable to realistically propel the mechanical queen with animatronic legs, so MPC grafted digital legs onto the models in postproduction.

Similarly, Anderson's vision of the Alien Queen as a rampaging T-Rex-sized monster could not be achieved solely with a mechanical puppet, whose torso, tail, and legs were inoperable. Alternatively, MPC tracked and animated the torso, lengthened and bulked up the legs and tail, and enlarged the feet to make the body pieces anatomically consistent with her movement.

To craft the all-CG creatures, technicians at Soho Scan separately captured the geometry of the arms, legs, heads, and other body parts of the ADI-designed, full-size animatronic props and costumes, delivering 3D scans ranging from 50,000 to. 4 million polygons. Next, the MPC team imported the segments into Cyberware's CySlice software and assembled them, then exported the geometry as a polygonal Alias Maya file. In Maya, the artists cleaned and scaled the parts, and stitched them together to create the final models, using scripts written with Byron's Poly Tools from Sebastian Thiel to manage the surfaces. All told, the completed Alien Queen has 200,000 polygons; the Alien warrior, 85,000; and the Predator, 50,000.

For the grid pattern on the head of an Alien subspecies, artists painted separate displacement, blood, and opacity maps atop the base maps.

The modeling team then crafted multiple variations of each Alien and Predator species from these basic designs, and tailored numerous versions of each model to suit the animation or resolution requirements of the shots. For the wide shots framing hundreds of thousands of Aliens climbing the pyramid, the artists created four models of varying resolutions ranging from 250 to 2300 polygons. The Alien Queen required three high-resolution variants, each depicting her in successively worse condition as the battle raged on.

At render time, Pixar's RenderMan converted all the polygonal models to subdivision surfaces, which offer the continuity of polygonal surfaces and the flexibility of NURBS, allowing for uniform deformation of the Aliens' skin. "Subdivision surfaces aren't perfect for every occasion, but for creatures they do quite well, generating a more intuitive topology for the skin deformation," says Valdez. "Points can be laid out in such a way that they are the most advantageous for simulating flesh and skin stretching and wrinkling."

In the most terrifying scenes from previous Alien films, the cinematography evoked terror through the use of shadow and suggestion, or leaving the monster off the screen altogether with the terror mounting in the audience's imagination. According to Valdez, when the Alien was explicitly shown in the past, it usually strained credibility. Yet for this film, Anderson wanted to show the creature in some of the more "heroic" poses from the comic books. Therefore, the team had to eliminate the awkwardness of their movements, which were limited by the human inside, and instead take advantage of the mobility offered by the CG versions, but without compromising the audience's familiarity with the pre-established nature of the creature.

"Our biggest animation challenge was trying to give the world a new Alien that would be a departure from what's been seen before, but still make it feel like the same character," explains Valdez.

Because of the complex biomechanical composition of the monsters—an amalgam of flesh, machine, and insect—MPC was concerned about their deformation. "The original Alien suits are foam rubber, and it was how you'd light them that kept them from looking fake," says Valdez, "and this was true of their CG counterparts."

Another crucial aspect in making the creature withstand the scrutiny of close shots was the articulation of the Alien Queen's neck. Working in Maya, the crew rigged the neck for intensive deformation, ensuring it twisted like an intricate meshwork of slimy metallic structures and insect-like segmentations. Though the design of the Alien is such that neither the warriors nor the queen have traditional muscles, they do have muscle-like features, which were rigged using a combination of joint-based muscles and blend shapes.

This series shows the complexity of the Alien warrior in various stages, including the skeleton, with 400 joints and 50 IK handles (top left), with smooth-shaded geometry (top right), and with varying levels of hardware texturing (bottom two).

Because of her segmented design, the queen's movement was limited, especially in the arms, which created rigging and animation challenges. On the other hand, the tails for the Aliens in AvP have an almost prehensile ability through a complex spline IK system that was set up as a separate rig.

For the first time in the Alien series, several metal chains are appended to the Alien Queen's body that she can wield to snag objects. Because the animators needed to increase the length of the chain from shot to shot for dramatic effect, riggers designed another flexible spline IK system that allowed for the application of any size chain on a per-shot basis, while constraints and additional rigging enabled the chains to dynamically snag and attach to props.

Meanwhile, the rig for the Alien warriors comprised 400 joints and 50 IK handles, with the neck, back, and tail having three spline IK chains respectively. The Predator, receiving a much simpler rig overall, required spline IK chains: one for the back and another for the neck.

For the climactic showdown between the two warring races—and the centerpiece visual effect of the film—MPC staged an epic battle between the live-action Predators and 16,000 digital Aliens atop the Antarctic pyramid site. To accomplish this, the studio further developed Alice, its proprietary crowd-simulation software, which was previously showcased in the extensive battle scenes in the feature film Troy.

The team customized Alice to generate Aliens that could crawl, leap, and move their tails en masse or individually. The program has rigid-body dynamics built in, so the crowds could respond intelligently to their environment. To individualize each crowd member, artists applied various Pose Deformers to vary the geometry, textures, and movements, achieving the latter by modifying and blending the motions assigned to each member on the fly. The Pose Deformers for the crowd of Aliens climbing the pyramid included three variations for mounting big steps and three others for navigating small ones, two for walking and running up the slopes, one for tumbling down the pyramid, and 12 for jumping. These cycles stopped at the summit of the pyramid, where the crowd members were then keyframed for the fight scenes.

In the movie's pinnacle, a raging battle scene between the two creature species, The Moving Picture Company generated thousands of CG Aliens. Then, using Alice, the studio's proprietary crowd-simulation software, the artists sent thousands of Aliens on an

When scanning a creature as texturally complex as the Alien, MPC inevitably had to divide the detail between the geometry and the displacement maps. Since paintwork for CG creatures tends to adhere to different visual rules than those for on-set props, it wasn't necessary for the team to capture the texture detail when it scanned the character. "Instead, the artists simulated a base coat of color and displacements, and then painted on a 'slime' layer, so the shader could vary the slimy quality of the surface," explains Valdez. "This emulated how the skin of the Alien suits worked on set."

The multi-layered shading networks for the Alien warriors included color, bump, specular, reflection, ambient, occlusion, and displacement maps, but the driving force behind the wet, gelatinous sheen of their bodies was the displacement and specular channels. Each of these had two components: The specular channel comprised a standard specular map and a wet specular map, and the displacement channel featured a standard 8-bit map as well as a 16-bit High Dynamic Range Image (HDRI) map to accentuate the finest cracks and details in the queen's crown.

Then, by applying multiple layers in Right Hemisphere's Deep Paint 3D and Adobe's Photoshop, the artists executed all the texture painting at 4k for the prominent "hero" Aliens, and at 2k for the less-prominent crowd Aliens. For an Alien subspecies called the Grid Alien, which sports a deep grid pattern on its head, the artists painted separate displacement, blood, and opacity maps on top of the base maps. The group also painted numerous tail textures to fit a variety of lengths and reflect varying levels of damage.

For the base model of the Alien Queen, which shows her in prime condition, the artists applied the same shading network used for the warriors. However, for her two other models, portraying her in successive stages of deterioration, MPC used both the HDRI floating-point displacement map and a secondary displacement map—painted in Deep Paint 3D and composited above the HDRI map using Apple's Shake—for the deep cracks in the her crown.

Given the slimy and highly reflective surfaces of the creatures, seamlessly intercutting the live-action Predator costumes and the Alien puppets with their digital counterparts demanded pinpoint accuracy in mapping the position of on-set lights, achieved through an HDRI lighting setup for each effects shot. Meanwhile, the lighting and compositing teams combined those reflection renders with CG lighting and highlights, allowing them to preserve creative control until the very lasts stages of production by quickly adapting a creature to last-minute re-shoots or changes in a plate.

The catchphrase for the film is "No matter who wins, we lose." But judging by the quality of the creature work, MPC can be declared the winner in this alien battle.

Martin McEachern, a contributing editor for Computer Graphics World, can be reached at martin@globility.com.

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